A positioning device for a sliding mechanism includes slidably connected exterior plate, intermediate plate and interior plate, characterized by the exterior plate having a stop block; the intermediate plate formed with a through hole, including having a pivot element mounted pivotally adjacent to the through hole and having a projection extending through the hole to engage the stop block, thereby preventing sliding action between the exterior and intermediate plates, a resilient element having a free end in contact with the rear end of the pivot element; and the interior plate having a push element with cam face; when an external force applied on so as to cause sliding action between the interior plate and the intermediate plate results in abutment of the cam face against the resilient element, thereby deforming the resilient element and rotating the pivot element about its axis, thereby disengaging the projection from the stop block.